In a known type of digital television test signal generator, such as the Tektronix 1910, the various test signals are inserted into the composite color video signal in timed relationship to the sync and burst of the video signal. The test signals themselves are generated by reading digital data out of ROM under control of a clock signal at 4f.sub.SC, where f.sub.SC is the color subcarrier frequency (3.58 MHz in the case of the NTSC signal format), and converting the digital data to analog form by a digital-to-analog converter (DAC). In order for the test signals to be inserted in timed relationship to the burst of the video signal, the 4f.sub.SC clock signal must have a fixed phase relationship to the color burst. Generally speaking, it is desired that the clock signal be adjustable in phase relative to the color burst, e.g. in order to allow for pre-compensation for system delays and to accommodate test signals that are encoded in ROM on different modulation axes. In the case of the Tektronix 1910 test signal generator, the desired phase relationship is established using an analog phase-locked loop. Thus, a conventional sync separator identifies sync of the video signal and opens a window during burst time while the video signal is applied to a phase detector. The analog output of the phase detector is applied to the control input of a voltage controlled oscillator (VCO), and the output of the VCO is fed back to the other input of the phase detector. Thus, the phase detector generates an error signal representative of the difference in phase between the burst and the output of the VCO, and the output of the VCO is thereby brought into phase with the burst. The VCO is used to generate the 4f.sub.SC signal. The operation of bringing the 4f.sub.SC signal into phase with burst is known as genlocking.
Among the disadvantages of using a conventional analog phase-locked loop are that an analog phase-locked loop requires calibration and drifts with time and temperature.